I. Field of the Invention
The present invention relates to a method for constructing a composite powdered metal component.
II. Description of the Prior Art
In constructing components from powdered metals, a die having both upper and lower die halves is typically used to first press the component. The die halves are movable with respect to each other and form a cavity therebetween which corresponds in shape to the shape of the desired finished component.
In order to construct the powdered metal component, with the die halves separated from each other, the die cavity is filled with the powdered metal. Thereafter, the upper die half is positioned over the die cavity and the die halves are compressed together under high pressure. The compaction of the powders within the die cavity causes the metal powders to adhere to each other so that the compacted component maintains its shape upon removal from the die.
The compacted component is then sintered, hot pressed or hot forged to densify the part. Sintering is carried out at or near the liquids temperature and bonds the particles together while hot pressing or hot forging can be carried out at lower temperatures and densities the part at or near the liquids temperature of the metal powders. In doing so, the metal powder bonds together to form a metal component.
Many of these previously known powdered metal components are formed from powdered steel powdered iron or alloys of powdered steel and powdered iron. In order to increase the strength and hardness of such parts, one prior practice has been to add carbon to the powdered metal typically in the range of 0.3-1.0% by weight, which significantly increases the hardness and strength of the finished component.
One disadvantage of adding carbon to the powdered metal, however. is that the finished component cannot he welded consistently due to the relatively high carbon content. In many applications, however, it is desirable that the component exhibit the high strength of carbon steel and still maintain the capability of welding the component in its final installation.
For example, in a gear having a hub and an annular gear rings it is highly desirable that the inside diameter of the hub enjoy a high strength and rigidity of high carbon steel while other portions of the gear remain weldable In order to accomplish this, it has been the previously known practice to carborize the inside diameter of the gear hub by axially stacking a number of hubs and then flowing carbonized gas through the interior of the stacked hubs.
While this previously known practice of hardening the interior of the gear hub by forming a carborizing gas through the hub has proven effective, it is time consuming and relatively expensive to perform. Furthermore, this previously known method is effective only for increasing the carbon content along the interior of the gear hub. Conversely, this previously known method cannot be used for hardening other portions of the gear, for example, the axial end of a hub.
In still other applications. it is necessary that the powdered metal component have some porosity, and thus a lower density, in order for the part to accept certain coatings or treatments. Such increased porosity, however, usually weakens the overall part.